Set a seasonal calendar in your yard at solstice

At sunset on Sept. 22, I'll be in my backyard with a couple of stakes and twine, and I invite you to do something similar. With just those simple tools, you'll be able to see some of the fundamentals of the solar system and part of how we humans can tell time around the planet.

If it's clear weather as the sun is going down on any night around the 22nd, you'll be in business. Just pound a stake into the Earth. The shadow from the stake will be running due east no matter at what latitude your town may be. Then pound Stake No. 2 in the middle of the shadow of Stake No. 1. Next, connect the two stakes with a bit of twine. You now have an east-west line segment, tied off on each stake.

I performed this ritual in the front yard at my previous house -- just to double check it had really been laid out with respect to geographic compass points -- and I was rewarded with the knowledge that it was. But I've moved since then, and I want to do the same task in the yard of my "new" house, just to make sure it was laid out correctly back in 1949 when its foundations were poured. (I'll sleep better knowing my toes are pointed toward due geographic north in my bedroom. That's just me.)

Once you have the east-west line segment made of twine, you can use a carpenter's triangle to place a north-south line at right angles to your east-west line. Then you'll have the four geographic compass points represented on the ground: geographic north, south, east and west. (Although it amazes many people, magnetic compass points are entirely different from geographic ones, and they will have to be the subject of another column.)

If you preserve the east-west line segment you made on the Earth through the months to come, you'll have a way of telling seasonal time with respect to a physical line on the Earth. From now until the spring equinox in March, the sun will both rise and set to the south of your east-west line.

The absolute nadir of the whole experience will fall around Dec. 21, the shortest day of the year in the northern hemisphere. But, by the spring equinox, the sun will have "moved" gradually back on the horizon so that it rises and sets along your geographic east-west line. Then, in the sunny part of the year, sunset and sunrise will occur north of your east-west line and the days will be wonderfully long.

But there are several interesting problems that lie at the heart of telling time by the sun -- and also knowing where we are on the Earth by using sunlight as our guide. Sunrise, after all, occurs minute-by-minute, moving westward across the planet, in a continuous process that goes on and on. But even though sunlight continuously blesses the Earth in this changing way, we humans want to divide Tuesday from Wednesday, and separate 2:49 from 2:50, all with what lawyers call "bright lines."

Bright lines superimposed on natural and continuous processes involve compromises. The Greenwich Meridian Line in Britain and the International Date Line is an enormous imaginary line that circles the whole Earth. It's a kind of compromise, although a highly useful one. When you cross the line in the Pacific, the day of the week rather magically changes. The other part of the same line in Greenwich, Britain, is home to Greenwich Mean Time (GMT), which is the time you'll hear announced on BBC radio.

Greenwich is also home to the Prime Meridian, the "zero" of the system of longitude we agreed to use long ago. Here in the U.S., we live to the west of Greenwich in Great Britain. That's why our degrees of longitude are all demarked "west" on maps. In Russia, longitude lines are "east." Greenwich was an arbitrary place to plunk down the central or zero line of longitude, but it works.

This October marks the 125th anniversary of the establishment of the Greenwich Meridian -- an event that will be celebrated by amateur astronomers in Europe -- and by some of us in our own back yards.